I just upgraded my build environment to Kubuntu 12.10 in a virtual machine (in order to get the updated cmake that DD mentioned above), with Qt5 installed as binaries from the link cited above. Had a little trouble getting started with cmake, namely the same errors that the OP encountered.
Using update-alternatives to point to the Qt5 qmake did not, by itself, help; but starting cmake with :
cmake -G "Unix Makefiles" -DCMAKE_PREFIX_PATH=/opt/Qt5.0.0/5.0.0/gcc/lib/cmake/ -DCMAKE_BUILD_TYPE=Release sigil
(as the error message suggested) did the job.
As an aside, a quick test drive of the build went fine, and as an added bonus I noticed that the usual KDE-related quirks (such as crashing when dragging chapters around in the book view) seem to be gone.
I'm not sure what to do with the extra preview window either, but as I seldom use it, it isn't a big deal.